The game separates mods by the name of the contract. If two mods of the same airline have the exact same contract name, they won’t merge. Only the older entry will be offered.
If the mods use different contract names, both will be offered.
Have you subscribed to those big modpacks (European airlines etc) or only to single airline mods? Our suggestion is, not to use those large modpacks as those are very outdated.
If you find several mods of the same airline, check the upload/last update date. It would be enough to subscribe to the latest one which usually provides the complete fleet and newest templaes.
I have now removed almost all the older / duplicate airlines
I did notice a few others when I compared the Contracts and the Flights
Sadly the deleted / removed airlines resulted in some ? in the Flight Planner, but I have resolved that as well now by simply cancelling the contract, which in turn removes the flights
So far, so good
The only other duplicate now are contracts with colour options (c) and (w)
However I know what that is now